Heat generated, for example by electronic devices and circuitry must be dissipated to improve reliability and prevent premature failure. Techniques for heat dissipation can include heat sinks and fans for air cooling, and other forms of cooling such as liquid cooling. Depending on the application, the heat sinks can be made of metal, or ceramic materials, but some times also out of polymeric materials. The latter constitute typically thermal greases alike silicones and epoxides thermal interface materials, used typically to adhere the circuits into the device structure itself. When it comes to for example casings of such devices, also thermoplastic thermal composites are used for the overall thermal management throughout the device. The increasing use of polymer materials is based on simple facts of reducing the device weight, and its cost. Moreover, thermally conductive plastics typically boast lower coefficients of thermal expansion (CTE) than for example aluminum and can thereby reduce stresses due to differential expansion, since the plastics more closely match the CTE of silicon or ceramics that they contact. Polymer composites offer also design freedom for molded-in functionality and parts consolidation; and they can eliminate costly post-machining operations. The use of polymeric materials is however limited by their native thermal conductivity properties, reaching typically thermal conductivity values of only around 0.2 W/mK.
For example, miniaturization of electronic chips has become an important topic for development of integrated circuit. Because sizes of electronic elements become smaller, and their operating speeds become faster, how to dissipate the heat generated by an electronic element during operation so as to maintain its working performance and stability has become one of the points for research.
In prior art, several methods to improve thermoplastic polymer thermal conductivity properties are presented. The methods include the use of boron nitride, alumina, graphite, boron carbide and other ceramic materials as additives for improving the thermoplastic polymer thermal conductivity.
Typically, thermal conductivity is measured both in-plane and through-plane of the material, the in-plane conductivity featuring normally higher thermal conductivity values than the through-plane conductivity.
The electric properties of thermal composites can be tuned by selecting either dielectric or electrically conducting filler additives. Typically, the additive total concentrations start from 20% and can be more than 80%. Some of the most advanced thermal composites can contain several of above mentioned fillers.
There are upper limits on present thermoplastic thermal conductivities, and it is difficult to improve these further due to already extremely high filler contents. Excess filler content is detrimental for the polymer composite other important properties, such as mechanical properties.
Therefore there is a need to improve thermoplastic thermal conductivities without increasing the amount of the filler.
Nanodiamond (ND) also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD) is a unique nanomaterial which can easily be produced in hundreds of kilograms by detonation synthesis.
Detonation nanodiamonds (ND) were first synthesized by researchers from the USSR in 1963 by explosive decomposition of high-explosive mixtures with negative oxygen balance in a non-oxidizing medium. A typical explosive mixture is a mixture of trinitrotoluene (TNT) and hexogen (RDX) and a preferred weight ratio of TNT/RDX is 40/60.
As a result of the detonation synthesis a diamond-bearing soot also referred to as detonation blend is obtained. This blend comprises nanodiamond particles, which typically have an average particle size of about 2 to 8 nm, and different kinds of non-diamond carbon contaminated by metals and metal oxide particles coming from the material of the detonation chamber. The content of nanodiamonds in the detonation blend is typically between 30 and 75% by weight.
The diamond carbon comprises sp3 carbon and the non-diamond carbon mainly comprises sp2 carbon species, for example carbon onion, carbon fullerene shell, amorphous carbon, graphitic carbon or any combination thereof.
For isolating the end diamond-bearing product, use is made of a complex of chemical operations directed at either dissolving or gasifying the impurities present in the material. The impurities, as a rule, are of two kinds: non-carbon (metal oxides, salts etc.) and nondiamond forms of carbon (graphite, black, amorphous carbon).
The use of nanodiamonds in any kind of polymers for thermal management use is currently very limited, and restricted to certain silicone material. Document US 2010/022423 A1 discloses use of nanodiamonds to increase thermal conductivity in polymeric grease. The nanodiamond thermal grease comprises a nanodiamond powder, a thermal powder and a substrate. The nanodiamond powder has volume percentage of 5% to 30%, the thermal powder has volume percentage of 40% to 90%, and the substrate has volume percentage of 5% to 30%. The nanodiamond powder and the thermal powder are distributed uniformly in the substrate to form the nanodiamond thermal grease.
Based on above, there is an emerging need to improve the thermal conductivity of also all the thermoplastic polymer materials, when ever they are used in applications where heat is generated.
The thermoplastic materials can be divided into three main categories, namely standard polymers, engineering polymers and high performance engineering polymers. Moreover, all these categories can be divided into two further morphological sub-groups, namely amorphous, semi-crystalline and crystalline thermoplastic polymers. All these materials also vary in respect to their lipophilic or hydrophilic properties, determined through their polymeric chain structure and contained functional groups therein. The mixing of possible fillers is restricted only to powder form filler materials, the highly viscous thermoplastic materials have to be melt into very high temperatures prior processing, and are not miscible with any solvents. If the added filler materials cannot be distributed evenly into the polymer matrix, but are forming heavy agglomerates in the produced matrix, the use of additives may also result in poorer mechanical and thermal properties as in initial, native polymer material. This problem gets more and more severe, the higher the total content of various fillers in a ready polymer composite is rising.